Elliott, welcome to the forum. These guys can tell you alot about carbines so you are in the right place.

Post some pictures and these guys can tell you the story of your carbine. The more detailed the pics, the more detailed the story.

As far as the finding a 12/42 barrel, it may prove to be difficult. You may want to check with Eddy Yuja who goes by 81mm on GB. His parts are the real deal. Stay away from a certain seller in Riverbank, California. While I have never bought from him, his reputation is that he'll sell you anything you want, but it won't be real...

Just my two cents....